1. Field of the Invention
The present invention relates to a process for forming a pattern of a fluorinated resin or amorphous carbon layer, more specifically to a process for forming an insulating layer in multi-layer wiring boards for semiconductor devices and multi-chip modules (MCM).
2. Description of the Related Art
Due to an increase in the integration degree and operation speed of semiconductor devices and MCMs, the insulating materials used in multi-layer wiring boards are being reconsidered. At the present time, SiO.sub.2 is used as the insulating material for semiconductor devices and polyimide is used as the insulating material for MCMs. Since these insulating materials have high dielectric constants, 4.0 (at 1 MHz) of SiO.sub.2 and 3.5 (at 1 MHz) of polyimide, the problems of the wiring signal propagation delay (wiring delay) and crosstalk are worsened. The wiring delay t is represented by t=.sqroot..epsilon./C where .epsilon. denotes the dielectric constant of an insulating material for a signal line and C denotes the speed of light and the wiring delay is elongated in relation to the square root of the dielectric constant of the insulating material. Accordingly, an insulating material having a low dielectric constant is necessary to shorten the wiring delay. The crosstalk is caused by the parastic capacitance between wiring layers. An insulating material having a low dielectric constant is also necessary to lower the parastic capacitance. Benzocyclobutane (BCB) has been proposed to replace SiO.sub.2 and polyimide, but this still has a high dielectric constant of 2.70 (at 1 MHz).
Fluorine-based or -containing resins or polymers are promising as an insulating material for wiring layers since they have a low dielectric constant. For example, polytetrafluoroethylene (P-TFE) has a dielectric constant of 2.2 (at 1 MHz), the lowest among organic materials.
The fluorine-containing resins or polymers are however difficult to form into a layer or film since they are not soluble in most solvents and therefore cannot be coated as is done for normal photo-sensitive resins. Some fluorine-containing resins which can be spin coated are commercially available, but these have poor adhesion to a substrate and are easily peeled off when heated. JP-A-3-80541, published in April 1991, discloses a process for forming a fluorine-containing resin layer, but this layer does not have a sufficient adhesion to a substrate.
There is a report in that if a P-TFE layer is deposited by plasma enhanced polymerization, the obtained layer has a high adhesion to a substrate. Nevertheless, the plasma polymerized P-TFE layer contains unsaturated carbon bonds and carbon radicals, so that the layer has a poor thermal resistance and does not have a low dielectric constant (about 2.8 at 1 MHz was reported). JP-B-3-62792, published in September, 1991, proposes a process of lowering the content of the unsaturated carbon bonds and carbon radicals by exposing a deposited carbon-containing layer to a fluorine-containing plasma gas (see also U.S. Pat. No. 4,902,529). In this process, however, only a thin surface region (about 0.30 .mu.m thick) of the layer can be fluorinated and C--C bonds may also be cleaved by fluorine radicals, so that the resulting layer is not a sufficient fluorinated layer with a high thermal resistance.
The object of the present invention is to provide a process for forming a fluorinated resin or amorphous carbon layer which can be advantageously applied as an insulating layer for wiring layers in semiconductor devices and MCMs.